For the study of flow past immersed objects, a channel with slotted sides and bottom and surrounded by an open chamber has advantages over the usual solid-walled channel. Previous experience has shown that the stream can be disturbed by self-induced oscillations. Experiments are described which demonstrate that the waves in the channel move upstream in synchronism with longitudinal stationary waves in the outer chamber. An approximate theory of this effect is derived. The trouble can be avoided by making the outer chamber only slightly deeper than the channel. The flow may be disturbed by self-induced oscillations. The waves are shown to move upstream in synchronism with stationary waves in the outer chamber. They can be eliminated by making the depths in the channel and the outer chamber nearly the same.